Deep soil carbon storage and microbial functioning in Amazonian alluvial forest soils

Amazonian floodplain soils are highly dynamic and support diverse ecosystems, yet the influence of contrasting forest types on deep soil carbon storage and microbial functioning remains poorly understood. This study provides a depth-resolved comparison of soil fertility, carbon storage, and microbial functioning across five contrasting Amazonian floodplain forest types. We compared soil chemical, physical, and microbial characteristics to 200 cm depth in five forest types on Ilha das Cinzas in the Amazonian floodplain: a mangrove forest, lower and upper forests, a regenerated forest, and açaí agroforestry. The sampling design comprised 15 sampling sites (three sites per forest type), with soil collected at four depth intervals (0–50, 50–100, 100–150, and 150–200 cm). The analyses included chemical properties, soil organic carbon (SOC) and SOC stock (SOCS), microbial indicators (β-glucosidase activity, BG, and microbial biomass carbon, MBC), soil moisture, and bulk density. SOC and SOCS significantly differed among forest types and depths, with the highest values found in mangrove soils (2.18% and 52 Mg C ha⁻¹). SOC was positively associated with MBC (β = 0.567, p < 0.001), while pH showed a significant negative association with MBC (β = −0.341, p < 0.001). These findings indicate that forest type and soil depth are strongly associated with variation in carbon storage, nutrient availability, and microbial functioning across Amazonian floodplain soils. Mangrove soils consistently exhibited greater carbon storage than the other forest types. Ecologically, these results highlight the role of mangrove and other carbon-rich floodplain forests in maintaining deep soil carbon reservoirs and associated soil functions; from a management perspective, they support the conservation of mangroves, the protection of carbon-rich floodplain ecosystems, and the inclusion of deep soil horizons in carbon accounting, soil-fertility assessment, and ecosystem-management strategies across Amazonian estuarine landscapes.